1. Field of the Invention
The present invention relates to a method of synthesizing diamond, and more particularly, it relates to a method of preparing a diamond film having a large area, which can be applied to a semiconductor material, an electronic component, an optical component, a cutting tool, a wear-resistant tool or a precision tool. The present invention also relates to diamond which is applicable to a substrate for a semiconductor device, a heat sink having high thermal conductivity or an optical component having a high light transmission property.
2. Description of the Background Art
Diamond has the following characteristics:
a. It has high strength.
b. It is excellent in wear resistance.
c. It has low compressibility and thermal expansivity.
d. It has extremely high thermal conductivity although it is an insulator.
e. It has a high refractive index, and is optically transparent with respect to ultraviolet, visible and infrared rays.
f. It has excellent chemical resistance.
g. It is excellent in sound wave propagation velocity.
h. It can provide semiconductivity when doped with a specific impurity.
In consideration of such characteristics, utilization of diamond in various fields is expected. Diamond is now regarded as a necessary and indispensable material in the industrial world.
A method of vapor-depositing diamond on a non-diamond substrate has recently been studied. In case of growing diamond on a non-diamond substrate, nuclei consisting of diamond grains are first formed on the substrate. Then, growth of diamond starts from the nuclei. In such vapor deposition of diamond, diamond is vapor-deposited in consideration of atomic arrangement on the substrate surface. When the surface of the substrate is oriented along a {100} plane, for example, diamond is grown on this substrate under such a condition that the diamond growth rate on the {100} plane is higher than those on the remaining planes. Methods described in the following literature references are known for growing diamond on non-diamond substrate:
(1) Koizumi et al. Appl. Phys. Lett. Vol. 57, No. 6 (1990), pp. 563-565
(2) Jiang and Klages, Diamond and Related Materials, No. 2 (1993), pp. 1112-1113
(3) Stoner and Glass, Appl. Phys. Lett. No. 60 (1992), p. 698
(4) Fujita et al., Summaries of Lectures in the Fourth Diamond Symposium (1991), pp. 13-14
(5) Inuzuka et al., Extended Abstracts (the 43rd Spring Meeting, 1996), the Japan Society of Applied Physics and Related Societies, No. 2, p. 403
(6) Yugo et al., Appl. Phys. Lett. Vol. 58, No. 10 (1991), pp. 1036-1038
Substrates consisting of cubic boron nitride, silicon and silicon carbide are employed in the methods of the above references (1), (2) and (3) respectively. Substrates consisting of nickel and copper, platinum, graphite, beryllium oxide or iridium are employed in the methods of the above references (4) and (5) respectively. A substrate consisting of silicon, silicon carbide or iridium is employed and subjected to negative biasing in the method of the above reference (6). In this negative biasing, the substrate is supplied with a low potential with respect to plasma in microwave plasma CVD (chemical vapor deposition). An effect of ion bombardment or radical concentration is proposed as a reaction mechanism for forming diamond in the method of reference (6). However, the reaction mechanism is not yet understood in detail.
In order to homogeneously form nuclei on the substrate by this method, plasma must be homogeneously generated and a uniform electric field must be generated on the surface of the substrate. However, it is difficult to implement such conditions, and hence diamond cannot be sufficiently grown on the substrate by the method of reference (6).
In relation to the method of reference (6), it is known that a large number of nuclei are formed when the strength of the electric field on the substrate surface is increased. If the strength of the electric field is excessive, however, it is difficult to obtain diamond of high quality due to formation of irregularly oriented diamond nuclei. On the other hand, a method of growing diamond on a substrate by thermal filament CVD is described in Chen et al., Appl. Phys. Lett. Vol. 67, No. 13 (1995), pp. 1853-1855.
This method succeeds in forming nuclei of diamond by supplying the substrate with a low potential with respect to filaments. In this method, radicals are conceivably formed by thermal filaments to generate dc (direct current) plasma by dc biasing.
Also in this method, however, it is difficult to uniformly generate an electric field on the substrate surface, and a problem has been reported whereby nuclei are formed only on an edge portion of the substrate. A problem whereby the discharge between the substrate and the filaments is destabilized has also been reported.
Japanese Patent Laying-Open No. 63-30397 (1988) describes a method of synthesizing diamond by extracting electrons from a thermoelectron-emitting material and discharging the same for generating plasma in the vicinity of a substrate. Also in this method, however, nuclei of diamond are so insufficiently grown that it is difficult to obtain high-quality diamond having a large area.